Calcium homeostasis in bovine somatotrophs: calcium oscillations and calcium regulation by growth hormone-releasing hormone and somatostatin

Abstract

The free intracellular calcium ion concentration ([Ca 2+] i) was measured in single cells of a population containing 65–80% somatotrophs, using the fluorescent Ca 2+-indicator Fura-2 and digital imaging microscopy. Spontaneous oscillations in [Ca 2+] i ranging in frequency up to 1.5 oscillations per minute were observed in 30% of somatotrophs. These Ca 2+ oscillations were blocked by the Ca 2+ channel blocker CoCl 2 and were thus proposed to be the result of influx of Ca 2+ into the cell, possibly as the result of spontaneous electrical activity [1]. GHRH (10–100 nM) increased [Ca 2+] i in 61% of the cells studied, although the amplitude and dynamics of the response varied from cell to cell. Typically [Ca 2+] i rose from 170 ± 26 nM to 321 ± 44 nM (n = 13) in response to a challenge with 66 nM GHRH. GHRH also increased the frequency of Ca 2+ oscillations in a number of cells, and some previously quiescent cells showed Ca 2+ oscillations following addition of GHRH. Forskolin, which raises cAMP levels in bovine anterior pituitary cells, also stimulated a sustained rise in [Ca 2+] i in 10 out of 14 cells tested. Somatostatin (SS) (10–80 nM) rapidly reduced basal [Ca 2+] i, blocked Ca 2+ oscillations, and blocked the [Ca 2+] i response to GHRH. The Ca 2+ channel blocker CoCl 2 (4 mM) had similar actions on [Ca 2+] i to those of SS. These results suggest that GHRH and SS may regulate GH release by modulating Ca 2+ entry into the cell through the cell membrane. The [Ca 2+] i oscillations seen in a proportion of the somatotrophs were modulated in frequency by GHRH and SS, and are probably generated by influx of Ca 2+ through channels in the cell membrane. Thus GH secretion may be regulated by changes in the mean level of [Ca 2+] i, which in turn, may be influenced by the frequency of [Ca 2+] i oscillations in bovine somatotrophs.